A new, dechlorinating bacterium
Several industrial activities of the previous decades resulted in serious contamination of groundwater. For instance, polyvinyl chloride (PVC) production and related activities cause annual underground releases of 137 tonnes of 1,2-dichloroethane (1,2-DCA) in the USA (1988-1999). The latter molecule has an environmental half-life of about 50 years, is the most abundant groundwater pollutant of all chloroethenes and –ethanes, and is a suspected carcinogen. Its physico-chemical properties result in a slow (bio)degradation in groundwater and a possible threat for humans and wildlife. For this type of contamination, no detoxification technology has been reported that is compatible with the mostly reductive conditions of groundwater.
The aim of this work was to search for bacteria that are able to rapidly, completely, reductively and metabolically dechlorinate this problematic molecule. In addition, other chlorinated alkanes such as 1,2-dichloropropane were envisioned. Some bacteria are currently known to dechlorinate some of these compounds in cometabolic reactions, resulting in (mostly) incomplete, uncontrollable and very slow dechlorinations. Only 1 isolate is known to rapidly dechlorinate 1,2-DCA, but its nutritional requirements are not defined and vinyl chloride is a toxic co-product of the reaction. In conclusion, bacteria that can be applied for efficient detoxification of important chlorinated pollutants as 1,2-dichloroethane, do not exist.
In order to find applicable bacteria for this environmental problem, a strategy was followed that differed from earlier enrichment experiments in two ways. First, the chlorinated alkanes, with 1,2-DCA as a model compound, were used as the only electron acceptors of selective enrichment media. Second, the starting bacterial inoculum was very special. It was obtained from the soil matrix of an anoxic water-saturated layer at 1 m depth, that had been exclusively polluted with 1,2-DCA for about 30 years.
A lucky combination of applying electron and carbon sources, allowed to detect unreported dechlorination characteristics which warranted a complete and fast dechlorination of 1,2-DCA. There was no link with any known bacterium.
Several hurdles prohibited a smooth and efficient isolation of this special bacterium. However, the use of supernatant in growth media combined with the dilution series technique, allowed to isolate the dechlorinating bacterium, designated strain DCA1, and to maintain and study it in pure culture.
It became clear that the bacterium respired the pollutant 1,2-DCA like humans respire oxygen. This respiration delivers energy, and allows the organism (bacterium or human being) to live and to reproduce. Physiological, morphological and phylogenetic characterization revealed that strain DCA1 was a new species of the genus Desulfitobacterium. It was proposed to designate the bacterium as Desulfitobacterium dichloroeliminans strain DCA1. Some 14 weeks after its isolation, the complete growth medium of the isolate was elucidated. Strain DCA1 was now the first bacterial isolate that completely dechlorinates some chlorinated solvents in entirely defined growth media. This growth medium definition allows very accurate steering of cell mass production by fermentation. Furthermore, an exclusive and stereoselective dichloroelimination reaction was investigated on a new range of chlorinated substrates, catalyzed by a new dehalogenating enzyme present in strain DCA1. With this enzyme, the first stereochemical analysis of anaerobic microbial chloroalkane dechlorination could be performed.
In contrast to all currently known dehalorespiring anaerobes, strain DCA1 does not convert nor produces any saturated chlorosubstrates. These findings indicate that a new biochemical pathway is present that can be the subject of fundamental research in the near future. Furthermore, the exclusive dichloroeliminating dehalogenase may have considerable impact on the elucidation of the origin and evolution of dehalorespiring bacteria. At this moment, it seems that the dechlorinating capacity of this bacterium has evolved within some decades. The latter is not that surprising, since the evolutionary speed of a bacterium is about 10000 times faster than that of mankind. Hence, humans need some 300000 years of evolution, where bacteria only need 30 years.
Injection of strain DCA1 into contaminated groundwater might be a cheap but efficient remedation strategy. The bacterium has shown powerfull detoxification characteristics in laboratory tests on groundwater. Since december 2002, successful in situ pilot applications have confirmed that injected living cell mass of strain DCA1 is also actively degrading in the underground. Moreover, this degradation is complete (almost no pollutant remains) and amazingly fast (days to weeks). Untill now, the only solution for anoxic groundwater contaminated with chloroalkanes was a costly and long-term pump-and-treat remediation (years).
The Avecom group recently has bought the patent rights on this bacterial strain for optimization of cell mass production and full-scale groundwater remediation. Several problem holders have shown interest in the new chloroalkane degradation technology.
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